1. Field of the Invention
The present invention refers to a doctor blade comprising a metal support completely coated with a polymeric material, in particular Teflon s®, and a method of coating said support.
Doctor blades are well-known tools and are normally used to remove from a cylindrical surface—with a finite radius (cylinder) or an infinite radius (flat surface)—a product (liquid, paste or powder) previously adhering to said surface.
Doctor blades can therefore be used in many fields to clean the surface of cylinders used (for example) for printing, for distributing and spreading adhesives, for grinding, etc.
In many printing methods doctor blades are used to distribute printing ink on a frame (such as, for example, in screen printing) or to remove excess ink from a printing cylinder (rotogravure, flexographic printing, etc.) or from a flat processed block (letterpress printing, tampography).
With particular reference to printing presses, a doctor blade operates in direct contact with the surface of the printing cylinder to eliminate the printing ink from the unengraved parts of said surface and the excess ink from the engraved parts of said surface; both the doctor blade and the surface of the printing cylinder are therefore subject to wear since the speed of rotation is high.
Consequently, printing cylinders are often coated with a layer of hard material (for example ceramic material or hard chromium), which can differ according to specific applications.
2. Description of Related Art
The doctor blades for printing presses currently in use mainly consist of a precision steel band, cold rolled, hardened and tempered, in which one or both of the straight edges destined to come into contact with the surface of the printing cylinder have the profile (rectangular, rounded, pre-sharpened with a foil shape or a bevel shape, etc.) considered most advantageous to meet specific requirements on a case by case basis.
To improve the resistance to wear of the edge of the doctor blade without (excessively) damaging the surface of the cylinder, the edge of the doctor blade is coated with thin layers of metal or with chemical nickel (possibly with hard substances added) or thin layers of carbides and/or of metal nitrides or relatively thick layers of ceramic material (metal oxides) are deposited thereon through long, costly procedures.
In order to avoid corrosion phenomena, the doctor blade in many applications consists of an extremely high-cost stainless martensitic steel band.
Doctor blades totally consisting of synthetic materials to improve their adhesion to the surface of the cylinder and to reduce the wear on said cylinder are also known to the art.
The various coatings are applied with methods known to the art, amongst which are spraying, rolling, dipping, powder coating, coil coating or electrolytic deposition (for example of nickel or chromium), etc.
Purely by way of non-limiting example some documents having as their subject matter coated doctor blades are cited.
US-A1-2004/0137261 describes a doctor blade of the type previously described, in which all the coating layers placed on the support (generally steel) are metallic, consisting in particular of electrodeposited nickel with various materials added.
NL-A-9300810 describes a doctor blade wherein the coating layer generally consists of a layer of plastic material, having a smooth surface, which can be different from or coincide with an elastic material fixed beneath the plastic material.
DE-U-20216494 describes a scraper ring (of metal or of carbon fibre) wherein the coating layer placed on the active surface consists of an elastic material chosen among the organic polymers and has a minimum thickness of 1 mm.
US-A1-2002/0157548 describes a doctor blade wherein the coating layer placed on the support (of cold-rolled steel with specific characteristics of composition and hardness) consists of a material with low resistance to wear (preferably) chosen among the metals, or among alloys, oxides, polymers or mixtures thereof; the application methods mentioned are manifold, amongst which there are plasma spraying, galvanising, etc.
US-A1-2005/0089706 describes a doctor blade of the type previously described, wherein the first coating layer placed on the support (of unspecified nature) is always metal (in particular, chromium plating or nickel plating) and the second layer consists of a deposition of organic resin.
None of the doctor blades currently in use, including those described in the above documents, has proved to be able to fulfil the users' requirements completely.